Another witness, Annisa Wynn, told KTLA-TV Channel 5 that the explosion occurred about two minutes into the show. Fireworks suddenly began shooting sideways along the ground into the crowd instead of up in the air, she said.

"The fireworks were so close, you could see them on the ground," Wynn said. "It never happens this way. People were screaming."

Amid the chaos, Ventura County firefighters set up a triage center in the park, where they treated adults and children, many of them hit by flying debris. Some people were taken by ambulance to area hospitals.

Most of the injuries were described as shrapnel-like injuries or burns. No fatalities were reported.

Taken to the hospital were four people with moderate to severe injuries and 16 with minor injuries. The rest were treated and released at the scene.

Simi Valley Police Cmdr. John Parks said the Fourth of July event was heavily staffed with police, park rangers and firefighters who were able to treat the injured almost immediately after the explosion.

Parks also said the public cooperated with authorities and quickly evacuated the park after the blast.

Jai Kohli, 30, of Pasadena, said there were about 10,000 people at the show and there was a lot of confusion after the explosion occurred. (Authorities said the crowd was estimated at 8,000 to 10,000.)

He recalled that one of the fireworks exploded low to the ground with a lot of force. "People around me instinctively ducked," he said.

Hazciel Vidrio, 26, of Simi Valley, said he and his family have attended the event the last several years, but "this was completely unexpected."

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After four or five rockets went straight up, he said, there was a loud blast and he realized that something was wrong.

"A lot of kids were screaming and crying," Vidrio said. "People were trying to get away from the fireworks as fast as they could and were running into each other."

[Updated at 11:55 p.m. July 4: Police Cmdr. Stephanie Shannon said there is no indication of foul play and that the explosion appears to have been an industrial accident. She added that the Ventura County Sheriff's Department bomb squad would remove the unexploded fireworks that remained at the park Thursday night. This post also includes updated injury totals and other details.]